1. Technical Field
This invention relates to apparatus and methods for reproducing two channel or multi-channel audio signals from a loudspeaker array. The invention is useful for stereo music reproduction and for reproducing surround sound audio program material that accompanies movies and television. The invention is an optimized configuration for the reproduction of two channel audio program material from closely spaced sources. The current invention includes an array of loudspeaker elements, generally (but not limited to being) centrally located with respect to a listening area, where the array is generally displaced toward the front of the listening area, and associated signal processing circuitry that allows the array to generate a spacious sound field while maintaining left(right imaging ability and a solid center image. The invention is capable of generating perceived sound source locations that are located far outside the array physical location. The perceived sound source locations are stable and do not degenerate as a listener turns his head or moves about the listening room. A user control is provided that allows the spatiousness and localization characteristics of the system to be adjusted by the end user.
2. Discussion
Typical stereo reproduction systems use two loudspeakers that are displaced to the left and right of a center listening axis for reproduction of a left and right stereo pair of audio signals. These systems are capable of generating virtual sound source locations that are generally limited to areas located between the two speakers. This is accomplished by adjusting the relative amplitude of a signal simultaneously presented to both channels. The virtual sound sources generated by controlling the relative amplitudes of the loudspeaker outputs do not remain stable throughout the listening environment. The images tend to collapse toward the near loudspeaker location as a listener moves off the center line between the two speakers.
Other systems have been constructed (Shivers.sup.1, Hafler.sup.2, Klayman.sup.3, and others) in an attempt to generate a more spacious sound field by adding various configurations of loudspeakers fed some form of difference signal (the difference between the left and right channel signals). It is generally acknowledged that the difference signal contains ambiance information, and that adding loudspeakers to the system that reproduce this signal can enhance the sense of spaciousness generated by the system. The addition of separate sources reproducing the L-R signal usually increases the sense of spaciousness, but it is often at the expense of left/right localization ability. The prior art systems do not attempt to control the radiation pattern of the different speaker systems in any way. The directions in which left and right channel signals, and difference signals, are radiated into space by systems that include these extra sources are random and un-controlled. These systems also require the use of additional loudspeakers to reproduce these difference signals, which increases their cost.
Still other inventors have tried to develop a centrally located loudspeaker array that is capable of generating an increased sense of spaciousness (Klayman.sup.3, Holl, Short et al..sup.4). These systems are designed primarily for use with video systems. These systems are capable of generating a spacious sound field but are not capable of achieving a strong left/right localization capability. They do not take into account the effect of element spacing, relative level, and relative phase between the array elements on the radiation pattern of the array. The net overall radiation pattern of these systems is not controlled and the ability of these systems to generate localization cues to simulate stable virtual sound sources located outside the physical array position is minimal. The effect of the interaction between the different array elements on the total radiated power of the array is not taken into account in these systems. The total power response of these systems is not controlled in any way.
Still other prior art systems have tried to extend the range of possible virtual sound source locations that can be generated by a stereo pair of loudspeakers by introducing interaural crosstalk cancellation. The intent is to obtain direct control over the signals presented to each ear of a listener and adjust them in such a way that the signals represent what would actually be at the listeners ears if a real source were located at the position of an intended location of a virtual source. Systems have been constructed to attempt this electrically using signal processing (Atal and Schroeder.sup.5, Cooper.sup.6, and others), or through the use of particular geometrical arrangements of loudspeakers (Polk.sup.7). These systems rely on the canceling of signals at a particular point in space that are generated by different physical sources. The cancellation that occurs is strongly dependent on the listening position and the orientation of the listeners head. The effect generated by all of these systems occurs for a single "sweet spot". The improved spatial performance degenerates rapidly with small changes in listener position or orientation. This degeneration does not occur for the present invention.
The crosstalk cancellation systems work by adding a slightly delayed and inverted version of the left channel signal to the right channel signal. By symmetry, a slightly delayed and inverted right channel signal is added to the left channel signal as well. The delay is calculated to be the time difference between the arrival of the signal at the ear closer to the source and the arrival of that same signal at the farther ear. Each signal is also equalized to take into account the effect of head diffraction. The intent of the processing is to cause the delayed left channel signal to arrive at the listener's right ear with exactly the same shape and at exactly the same time as the crosstalk signal from the left speaker, but inverted in polarity so that the two signals cancel. The same is intended for the left ear. It can be seen that the system relies on the precise timing of signal arrivals, along with the orientation and position of the listeners head, in order for the cancellation to work. The cancellation can only work over a relatively small area because of the precise timing of signal arrivals required.
There are some embodiments discussed in Cooper.sup.6 that superficially resemble some embodiments of the invention of this disclosure. Upon closer examination, they are found to be significantly different. In one embodiment, Cooper uses a monopole and a dipole speaker where the monopole is fed an equalized L+R (sum) signal and the dipole is fed an equalized L-R (difference) signal. The combination of a monopole and dipole speaker, where the monopole is fed an equalized sum signal and the dipole is fed an equalized difference signal also appears in the present invention. However, the equalization used in the present invention and that used in Cooper differ significantly. As a result, the behavior of the two systems differ significantly.
The equalization described by Cooper and others depends on the spacing between a listeners ears, and the angle of the loudspeakers with respect to the listeners head, and is designed solely to compensate for the diffraction of signals around the listeners head. The equalization used in the present invention however, depends solely on the physical spacing between the loudspeaker array elements. There is no dependence on the geometry of the listeners head whatsoever. As a result, the form of the equalization is different in the present invention than that required by the cross talk cancellation schemes, and the behavior of the systems is different as well.
The equalization used in the crosstalk cancellation systems is only concerned with the control of the direct sound arrival from the loudspeakers, at a particular point in space, to generate specific frequency responses at the location of the listeners ears. The crosstalk cancellation schemes are not concerned with radiation from the loudspeakers in any direction other than directly at the listener. The crosstalk cancellation systems do not attempt to deal with listening locations distributed throughout a listening room. The crosstalk cancellation systems are not concerned with the total power radiated by the combined loudspeaker elements. The crosstalk cancellation systems do not consider the effect of loudspeaker element spacing on the radiation pattern and total radiated power of the combined loudspeaker elements. The equalization shown in Cooper and described by others will cause significant coloration of sound, because of their failure to consider the radiated power and radiation pattern of the complete system.
The intent of the present invention is to use particular array configurations, and equalization that directly depends on the array configuration, to control the overall radiation pattern of the array in a specific fashion (which will be described later). The present invention is concerned with controlling the sound radiated in all directions from the loudspeaker array, not just directly at a specific listening position. The primary intent of the invention is to radiate different signals in different directions, to alter the reflected to direct energy ratio heard by listeners throughout the listening room. The reflected to direct energy ratio is controlled in an attempt to steer the localization of signals to the location of the reflections, away from the source of direct sound. It is also the intent of the current invention to provide a system that has a flat power response as a function of frequency over the frequency range where the radiation pattern of the array is being controlled. Controlling the radiated power of the system helps minimize frequency response aberrations throughout the listening area. The system radiated power remains flat, regardless of the adjustment of the spatial controls. (Spatial controls are described later as part of the overall discussion of the different embodiments of the invention. The function of the spatial controls is to alter the radiation patterns generated by the array in a useful manner, which is also described later.)
It will be shown later that the choice of element spacing is a trade off between efficiency at low frequencies and radiation pattern control at high frequencies. There are different embodiments that will use different element spacing for operation over different frequency ranges. The frequency response of the equalization required in the present invention will be shown to directly depend on desired operating frequency range of the array, which is directly determined by the array element spacing. This direct dependence of equalization on the orientation of the individual loudspeaker elements is of key importance in the present invention, and is not known in the prior art.
Still other prior art systems attempt to alter the reflected to direct sound ratio of the sound radiated from a single loudspeaker by using multiple radiating elements, where the majority of the elements are faced away from the primary listening position. An example of such a system is the Bose 901 loudspeaker, marketed by Bose Corporation. This loudspeaker uses a total of nine full range 4.5 inch transducers, where one transducer is pointed at the listening area and the other eight are faced away from the listening area. This system will be capable of increasing the reflected to direct sound ratio, but only at higher frequencies. At low frequencies, the loudspeaker will radiate omni directionally, as the sources are small compared to the wavelength of sound at low frequencies. The relative magnitude and phase of the different element outputs are not manipulated in any way in an attempt to control the radiation pattern at low frequencies. All the elements operate in phase over their entire operating frequency range. It will be shown later that the low frequency range is precisely the frequency range where the reflected to direct sound ratio needs to be controlled in order to generate localization cues that are displaced away from the physical location of the loudspeaker. It is precisely the directivity pattern of the loudspeaker array at low frequencies that is controlled in the present invention.